The logic is simple. The compound in the first compartment will be exerting some pressure, say P1, on the right side. Similarly the compound in the second compartment will be exerting some pressure, say P2, on the left side.

So, the final/ resultant pressure will be calculated by the difference of the two i.e., |P1-P2|.

The direction will be specified by the greater term among the two i.e, if P1 is greater than P2, the pressure will be exerted on the right side and similar for P2 > P1.

Hope this helps.